Contra- and ipsilateral auditory stimuli produce different activation patterns at the human auditory cortex

Auditory evoked magnetic fields were recorded over the right hemisphere of healthy humans The stimuli were noise bursts presented either to the contra- (C) or ipsilateral (I) ear in different combinations. The largest deflection of the responses, N100m (magnetic counterpart of electric N100), showed a field pattern which suggests activation of the supratemporal auditory cortex. In an oddball paradigm, where the standards (90%) were 400-ms noise bursts presented to the contralateral ear, and the deviants (10%) similar stimuli to the ipsilateral ear, the deviants elicited on the average 130% stronger equivalent dipoles for N100m than standards. Contralateral standards did not substantially decrease the response amplitude of ipsilateral deviants as compared with the response amplitude to ipsilateral stimuli alone presented at the interstimulus interval of the deviants. When two 50 ms noise bursts, separated by 310 ms, were presented once every 2 s, N100m evoked by the second stimulus of the pair was smaller when the stimuli were presented monaurally (C-C, or I-I) than to different ears (I-C or C-I). The results suggest that contra- and ipsilateral auditory stimuli are analyzed, at least in part, in different neural networks at the human auditory cortex.